A nuclear fuel element comprises fuel rods with cladding tubes filled with fuel pellets. It is known to provide cladding tubes with an internal layer of zirconium or a zirconium alloy to protect the fuel element from destruction caused by pellet-clad interaction, PCI, in case of rapid power increases.
SE 7810262 describes a nuclear fuel element with improved properties regarding resistance to PCI and comprising a composite cladding container comprising an outer part of a zirconium alloy and an inner part bonded to the outer part and consisting of zirconium with an impurity content to about 1000 to 5000 ppm. This composite cladding tube is manufactured by inserting a sleeve of sponge zirconium, which is to be used as an internal part, into a hollow billet of a zirconium alloy, which is to constitute the outer part, whereupon the unit is subjected to explosion bonding of the sleeve to the billet. Thereafter, the composite product is extruded at a temperature of about 538.degree.-750.degree. C. while using conventional extrusion methods. Thereafter, the extruded composite produce is subjected to a conventional tube production until the desired dimension of the cladding has been reached. The bond of the sleeve to the billet can also take place by means of heating to 750.degree. C. for 8 hours to achieve a diffusion bond. The manufacture of the composite cladding can also take place by extruding the unit, consisting of the inner sleeve and the outer shell, while using conventional technique.
EP 194 797 describes a cladding tube with an outer component and an inner component intended to protect the cladding against PCI damage. The inner component has a specific composition, zirconium with 0.4-0.6% tin, 0.5-1.4% iron and 100-700 ppm oxygen, in order to impart to the inner component good corrosion properties in the event that water should penetrate into the rod. The cladding tube is manufactured by processing the ingot of the inner component in a conventional way for the manufacture of Zircaloy, including beta quenching, before the inner component is joined to the outer component. After the joining of the outer and inner components, the cladding tube can be additionally beta-quenched either before the last cold-rolling step by beta-quenching the outer surface, or before the penultimate rolling step by beta-quenching the entire cladding wall comprising both the outer and inner components.
EP 155 603 also describes a nuclear fuel element comprising a composite cladding tube with an internal lining of zirconium to avoid cracking in the cladding upon thermal expansion of the pellets. According to this patent specification, it is known to reduce the sensitivity of the zirconium lining to cracking by limiting the total amount of impurities to a level below 5000 ppm and by maintaining the ratio of the oxygen content to the iron content greater than 1. The manufacture of a zirconium-lined cladding tube is performed by melting zirconium into an ingot which is then forged and shaped into a hollow billet. The hollow billet is inserted into another hollow billet of a zirconium alloy. The composite billets are heat-extruded into a tube blank. The tube blank is then subjected to repeated cold rolling operations and heat treatments according to conventional tube manufacture. A heat-treatment step consisting of a solution treatment at 800.degree. or 860.degree. C., near the phase transformation temperature to cause secondary phases in the material to become dissolved, and thereafter a rapid cooling to room temperature as well as a stress-removing heat treatment at a low temperature, for example 550.degree. C. for 2 hours, are carried out either after the forging or after the heat extrusion or as a last step on the finished tube, whereby it is considered to be especially effective to carry out the heat treatment as a last step. The best result is achieved according to the specification when the solution treatment is carried out in all the alternative steps during the process.
SE 8903595-0 describes a method for manufacturing cladding tubes of zirconium alloy, the outer surface of which is to obtain improved resistance to nodular corrosion during operation in a boiling reactor. The cladding tube is manufactured from a Zr-base alloy and is preferably beta-quenched before extrusion. Before the last cold-rolling step, the outer surface of the tube is beta-quenched. After extrusion, thus, only an outer part of the tube is beta-quenched in order to impart to this part improved corrosion resistance. Such a tube can also be provided with an inner component for PCI protection. After extrusion, the inner component in such a case will not be affected by the beta quenching prior to the last rolling step.
SE 8301770-7 describes a fuel rod comprising a composite cladding tube with an internal layer consisting of sponge zirconium alloyed with 0.1% tin. This fuel rod exhibits an increased resistance to the corrosive effect of water and water steam at elevated temperature. The cladding tube is manufactured by arranging a tube of the zirconium alloy, constituting the inner component, in a coarser tube of Zircaloy, the end surfaces of the two tubes being welded together. Thereafter, the composite tube is extruded without being subjected to any heating. The extruded product is then cold-rolled in several steps with intermediate recrystallization annealings at about 650.degree. C. and is finally annealed after the last cold rolling at about 525.degree. C.
The inner side of the cladding tube can be subjected to the corrosive effect of water and water steam in those cases where damage occurs on the cladding, which may result in water penetrating into the tube. The water will then become evaporated upon contact with the hotter fuel pellets. This results in the inside of the cladding tube being subjected to a corrosive effect of water/water steam of high temperature. When zirconium and zirconium alloys are corroded, hydrogen is formed, which to a certain extent is taken up by the cladding. The hydrogen will be precipitated in the form of zirconium hydrides. The zirconium hydrides are brittle and have a negative influence on the integrity of the tube. It is therefore of importance that the corrosion and the hydrogen absorption, caused by the corrosion, are as slight as possible in order not to risk that damage involving the penetration of water is extended such that a major quantity of water can penetrate through the tube and leaching of uranium dioxide and radioactive fission products can take place. Damage to a cladding tube can occur for several reasons, for example wear or defects in the welds.
The outer side of a fuel rod is always in contact with water and water steam, and it is known that heat treatments affect zirconium and the resistance of zirconium alloys to corrosion in water and water steam at a high temperature. Thus, a large number of publications describe heat treatments intended to improve the resistance of zirconium alloys to corrosion. EP 71193 describes how, during conventional manufacture, an ingot of a zirconium alloy is first forged in the beta-phase range and thereafter heat-treated in the beta-phase range followed by rapid quenching. Then, the billet is forged in the alpha-phase range and extruded, followed by tube manufacture involving cold-working steps and intermediate annealings in a conventional manner. To improve the corrosion resistance of the tube, it was found that the favourable solution treatment in the beta or alpha+beta-phase range followed by rapid quenching from the heat treatment temperature should be carried out also at a later stage in the process, after hot extrusion of the billet.